This invention relates to the field of pulp washing technology. Objects of the invention are a method of washing pulp by means of a gradually diluted wash filtrate recovered at an earlier stage of the washing, and a plant for carrying out the method.
The washing of pulpxe2x80x94the removal of dissolved, undesired components from the pulp following various delignification stagesxe2x80x94is still one of the most difficult operations even in the field of the most recent pulping technology, calling for continuous development of equipment and improvement of methods within the washing processes throughout the fibre line.
There are many kinds of washers in use, drum and Fourdrinier wire washers, presses and diffusers operating at various pressures.
The continuous aim being, for both economical and environmental reasons, to reduce the amount of water consumed per pulp ton, multistage washing methods and plants have been developed wherein the wash filtrate is re-circulated through the pulp at a process stage at which the pulp is less pure than the relevant filtrate. For example, in Finnish patent application 980481, a method of re-circulating washer filtrates is described, which method can be applied to a multistage drum washer. The pulp web to be washed is divided into zones from which the filtrate fractions are individually collected and returned in the counter-flow direction to serve as washing liquids in less pure zones. Multistage washing methods have been developed also for diffusers, as described in U.S. Pat. No. 4,705,600. A multistage wash is particularly difficult to carry out in pressure diffusers, as it is hard to avoid channelling in the pulp and intermixing of the filtrate fractions collected. In U.S. Pat. No. 5,567,262, a two-stage pressure diffuser is disclosed in which the filter unit is divided into two compartments and in which the wash filtrate of the less pure end is returned counter-currently. Further, U.S. Pat. No. 5,482,594 discloses a pulp washer operating at elevated pressure and having two main stages, in which the batch of pulp slurry is first dewatered on a wire table and subsequently carried to a washing position on the table. The recirculation of wash filtrates is also described, wherein filtrate fractions having different concentrations are collected in individual containers and wherein the number of washing stages can be five or more, depending on the number of collecting vessels and on the recirculation order.
The method according to claim 1 has now been invented for improving the efficiency of multistage washing, wherein the initial washing of a given batch of pulp is performed by means of a steplessly diluted, recovered filtrate fraction originating from the preceding batch of pulp, prior to the final washing of the pulp batch using available washing water or filtrate coming from the process. In the method, the recovered fraction filtrate is kept stored in such a way, that the concentration gradient of the dissolved substances is maintained until the fraction is used for washing the following batch of pulp, which is not possible in the conventional intermediate fraction tanks in any type of prior art multistage wash A further object of the invention is a pulp washing plant wherein the method according to the invention is carried out in a laboratory-scale test washer and applied to industrial pulp washing plant solutions.